An analyte of interest is often contained in the liquid phase of the material such as body fluid, to be analyzed. Particulate matter present in body fluids, needs to be removed prior to analysis, because material such as thrombocytes, erythrocytes and leukocytes in whole blood, may interfere with the usual analytical chemical techniques used in the determination of the presence and concentration of certain dissolved components, leading to an inaccurate result. For example, in the case of human blood analysis, most analytical tests need to be performed on the serum or plasma, and this first requires removal of the erythrocyte, which would otherwise interfere optically and/or chemically with the test.
Such problems resulting from the interference of particulate materials with the analytical technique and the methods utilized to remove the particulate material from the fluid being analyzed to overcome these problems, have been reviewed in Canadian Patent No. 1,322,335 as well as Canadian Patent No. 1,177,374. Traditionally, serum or plasma is separated from the erythrocytes by centrifugation, a process which involves expensive equipment and the use of reagents which may be hazardous to humans. In addition, this method requires relatively large quantities of blood, is manually intensive and is generally regarded as only suitable for use in laboratories by properly trained staff.
As such, there has been a growing interest in rapid, simple to use, disposable diagnostic devices which require only slight amounts of blood or body fluid, do not require laboratory facilities, and which can be used by non-chemists. These devices generally involve flow of the body fluid through a filtering medium and the use of lyophilized reagents (U.S. Pat No. 5,198,193, Bunce et al.). The more traditional centrifugation method for collecting fluids such as plasma free of particulates is incompatible with this type of technique.
The use of filtering methods compatible with porous media devices to provide for simple, inexpensive separation of small volumes of plasma has been disclosed in Canadian Patent No. 1,177,374 (Vogel et al., Boehringer Mannheim GmbH) as well as in Canadian Patent No. 1,322,335 (Koenhen et al., Primecare B.V.). Briefly, such devices usually consist of one or more separation layers, on an inert support. The separation layers serve to retain and thus separate the particulate material from the fluid being analyzed, i.e. blood. The fluid then contacts a collection/test layer or layers underneath where reagent(s) which react with the component(s) in the fluid to be determined, cause an observable and measurable change in a physical property (such as by a colour change), and thereby provide a quantitative or semi-quantitative result with respect to the components to be determined.
Canadian Patent No. 1,322,335 teaches use of hydrophilic microporous membranes of a defined pore volume at least one of which is the collector membrane. In a preferred embodiment is taught at least one asymmetric hydrophilic microporous membrane, separator membrane, wherein the body fluid is applied on that side of the asymmetric membrane which contains the largest pores. Thereby, the body fluid with particulate material penetrates immediately into the pores, the particulate material being retained in the pores as they become gradually smaller and the clear fluid penetrating further, to a collection membrane of defined pore volume. The assymetric collector membrane is removed prior to analysis.
One significant aspect of this preferred embodiment is that because of the asymmetric and open pore structure the fluid spreads out in all directions whereas the particulate material does not, whereby it is possible to separate that part of the membrane wherein the particulate materials are retained. This shall be described further herein below.
Filtering devices for use in association with disposable diagnostic devices that will not require laboratory facilities are disclosed in patents or are in commercial use. Canadian Patent No. 1,177,374 teaches a device with glass fibres to separate plasma from whole blood involving a removable glass fibre filter, a collection membrane for a defined volume of plasma and a hydrophobic barrier to prevent erythrocytes from reaching the analytical region. In the filter, there is lateral flow of erythrocytes and plasma. There is also disclosed a hydrophobic mesh to prevent liquid transfer until pressure is applied. There is a gap to prevent premature reaction of the plasma with the analytical reagents.
The Abbott Laboratories TEST PACK* device and Pacific Biotech CARDS* (*Trademarks) represent the use of removable filters for use with urine pregnancy tests and a cup-like holder for securing the filter membrane comprising two molded clamping forms. The holders are arranged to force the membranes into a disc-like form, the underside of which co-acts with the diagnostic membrane. A hydrophobic mesh prevents liquid transfer until the diluent is applied. The filter configuration is circular, and although the area of the filter membrane is larger than both the sample application site, and the area of contact with the analytic means, the additional annular area serves only as a means of retaining the filter.
The Johnson & Johnson LIFE SCAN* glucose strip device also teaches a filter membrane larger than the application site with adhesive to fix the filter membrane position onto the holder. This filter has a uniform pore structure, and it is thought, the large filter area is merely a convenient way of attaching the filter by adhesive.
Some analytes are present in relatively large quantities in blood and can easily be detected in a quantity of blood available from a fingerprick, typically 30.mu.l. Alternatively, some analytical methods are very sensitive, and again only require a small amount of body fluid, i.e. blood. However, the only way some analytes can be detected in meaningful amounts is by using more blood. The amount required may exceed that available from a fingerprick. Furthermore, removal of the erythrocytes using filtration methods becomes more difficult as the filtering capacity of the porous media depends largely on its surface area. If the area is small, the pores quickly become blocked. As the filter area requirement increases, the whole blood application site area and plasma outlet are also increased. Hence, using known constructions, it becomes difficult to apply blood evenly over a large area. It is also difficult to couple the large plasma outlet of the filter to the smaller area of the analytical strip. Furthermore, the whole device can become unacceptably large.
Canadian Patent No. 1,322,335 and the other references do not address the problem associated with known constructions of increasing the volume of plasma available for analysis, and in particular means of interfacing a large filter area to the blood application site, and to the analytical strip.
*Trademark